Among organoelectronic materials having a photoelectric function which produce electroconductivity or electric charges when being irradiated, most of low molecular weight organic compounds are incapable of forming thin film by themselves. Accordingly, when thin film is to be formed with such known low molecular weight organic compounds, they are dispersed in a binder resin (that is, diluted with a binder resin), and the resulting dispersion is applied onto a substrate to form thin film. Thus, the conventional organoelectronic materials comprised of low molecular weight organic compounds are influenced by the binder resin which forms a matrix, but also they are diluted with the binder resin so that they cannot exhibit sufficiently the properties that they originally possess.
In addition, if the conventional organoelectronic materials comprised of low molecular weight organic compounds form thin film that is relatively stable at normal temperatures with the aid of a binder resin, they have low glass transition temperatures so that the film is poor in heat resistance and is not suitable for use in practical devices. Accordingly, the development of organoelectronic materials that are capable of forming amorphous film at normal temperatures or more has been pushed on with in recent years.
On the other hand, as described in JP 6-1972A and 7-90256A, among a variety of electronic devices, an organic electroluminescence element in particular can be driven at a low voltage with high efficiency and at high luminance, but also it can be made thin because it is a self-emitting device. Hence, in recent years, the investigation to put the organic electroluminescence element to practical use as display devices as well as backlights or illumination devices is pushed forward.
The electroluminescence element is usually comprised of a transparent substrate such as a glass substrate having an anode made of a transparent electrode such as an ITO membrane (indium oxide-tin oxide membrane) laminated thereon, and a hole injecting layer, a hole transporting layer, an emitting layer and a cathode made of a metal electrode laminated on the anode in this order. The anode and the cathode are electrically connected with an external power source. In some cases, the hole injecting layer and the hole transporting layer are formed as a single layer, and in some cases, an electron transporting layer is laminated between the emitting layer and the cathode. Many other layer structures to form organic electroluminescence elements are known, as described in, for example, JP 6-1972A.
In such an organic electroluminescence element, the hole injecting layer adheres to the anode, and transports holes from the anode to the hole transporting layer, and the hole transporting layer in turn transports the holes to the emitting layer while blocking electrons, whereas the electron transporting layer adheres to the cathode, and transports electrons from the cathode to the emitting layer. Thus, when an electron injected from the cathode and a hole injected from the anode recombine in the emitting layer, light is emitted and radiated outside through the transparent electrode (anode) and the transparent substrate. It is already known that when the hole injecting layer (and the hole transporting layer) and the electron transporting layer are laminated in this way together with the emitting layer between the electrodes, the emission efficiency is improved.
As organoelectronic materials used in the hole injecting layer, hole transporting layer, or hole injecting/transporting layer, that is, organoelectronic materials used as hole injecting/transporting agents in the conventional organic electroluminescence elements, aromatic tertiary amines have been known, such as 4,4′-bis(N-(3-methylphenyl)-N-phenylamino)biphenyl (TPD), as described, for example, in JP 7-90256A, and 4,4′-bis(N-(1-naphthyl)-N-phenylamino))biphenyl (α-NPD), as described, for example, in JP 5-234681A, are known. However, the organic electroluminescence element in which these aromatic tertiary amines are used either as a hole injecting layer, a hole transporting layer, or a hole injecting/transporting layer is still insufficient in performance.
Besides, the aromatic tertiary amines mentioned above have low glass transition temperatures and insufficient heat resistance so that the hole injecting and/or transporting layer formed in the form of thin film with the aromatic tertiary amines are promoted in crystallization on account of heat generated when the resultant organic electroluminescence element is driven. Hence, the organic electroluminescence emission has a reduced efficiency, but also it may occur that the element is destructed, that is, the element is poor in durability.
Under these circumstances, tris(p-terphenyl-4-yl)amines have been proposed in JP 06-228062A as an organoelectronic material which can form amorphous film at ordinary temperature or more, and which in addition has a high glass transition temperature and is excellent in heat resistance. The tris(p-terphenyl-4-yl)amines have a high ionization potential so that they can be suitably used as an emission material as described in JP 07-53955A, however, the effective use of the tris-(p-terphenyl-4-yl)amines as a hole transporting agent has not been known thus far.
We have made intensive study to solve the problems involved in the conventional hole injecting and/or transporting agents, and the organic electroluminescence elements in which these agents are used, and as results, we have found out that the combinational use of an aromatic tertiary amine having an ionization potential in the range of 5.2-5.6 eV as a hole injecting agent and the above-mentioned tris(p-terphenyl-4-yl)-amines as a hole transporting agent provides an organic electroluminescence element which can be driven at a low voltage with a high efficiency and at high luminance, and thus we have completed this invention.